1. Field of the Invention
The present invention relates to a combination grinding machine adapted to sequentially grind a plurality of axially separated portions to be machined (hereinafter, called merely object portions) of a rotatably supported workpiece by means of a pair of cooperative grinding wheels which are controlled independently of each other to be fed in the axial and radial directions of the workpiece.
2. Description of the Related Art
A combination grinding machine of this kind; particularly, that for grinding a crankshaft includes a pair of wheel heads which each include a grinding wheel and which can be numerically controlled independently of each other to be fed in the axial and radial directions of the crankshaft, which serves as the rotatably supported workpiece. The pair of grinding wheels are controlled so as to efficiently and cooperatively grind a plurality of crank pins two at a time according to a machining program stored in a numerical control unit.
During grinding, the crankshaft is rotated about the axis of journals, so that the crank pins revolve around the axis of journals. The wheel heads advance and retreat synchronously with crankshaft rotation so as to advance and retreat according to the angular positions of the corresponding revolving crank pins.
Conventionally, in order to improve the profile irregularity of ground portions, a crankshaft which has undergone grinding undergoes lapping. The thus-completed crankshaft is attached to, for example, a cylinder block of an internal combustion engine for automotive use. For lapping, a separate lapping machine is employed. In the lapping machine, ground surfaces of the crankshaft are lapped by means of tape-shaped sand paper or polishing film.
Since grinding and lapping are performed by use of different machines, a crankshaft must be transported from a grinding station to a lapping station. Also, idle time consumed, for example, for positioning on a lapping machine is involved, thus impairing machining efficiency. Because of intensive wear of sandpaper or polishing paper, lapping involves a problem of the sandpaper or polishing paper breaking. Therefore, lapping itself is not highly efficient.
Further mover, although conventional lapping can improve surface roughness, the conventional lapping has the following drawback. That is, in the conventional lapping, sandpaper or polishing film is merely pressed against a crank pin via a floating mechanism without accurate control of the spatial relationship (i.e., relative position) between the sandpaper or polishing film and a crank pin. As a result, the lapping process impairs high roundness and cylindricity of crank pins which have been accomplished in the preceding grinding process.
A primary object of the present invention is to solve the above-mentioned problem involved in the conventional machining in which grinding and lapping are performed separately from each other.
Another object of the present invention is to prevent an impairment in geometrical accuracy of object portions in a lapping process subsequent to a grinding process.
Still another object of the present invention is to efficiently grind and superfinish a plurality of object portions of a workpiece.
A further object of the present invention is to prevent unfavorable events involved in grindingxe2x80x94such as deflection of a workpiece derived from a mechanical contact between a grinding wheel and an object portion and vibration of the workpiece and machinexe2x80x94from having an adverse effect on final finishing performed on ground portions.
An additional object of the present invention is to perform highly efficient grinding without involvement of deflection of a workpiece in a grinding process, which involves a relatively heavy machining load, and to perform highly accurate final-finishing without involvement of rubbing of a ground surface against a mechanical member other than contact with a final-finishing tool in a final-finishing process, which involves a relatively light machining load.
A still further object of the present invention is to supply a sufficient amount of grinding solution to a region of grinding during grinding so as to prevent the occurrence of burning on a ground surface, and to eliminate an adverse effect of a dynamic pressure of grinding liquid supplied to a region of grinding during superfinishing so as to attain high finishing accuracy.
A still further object of the present invention is to enable efficient grinding and superfinishing of a plurality of journals and crank pins of a crankshaft through a single setup of the crankshaft on a grinding machine.
To achieve the above objects. the present invention provides a combination grinding machine comprising a first wheel head and a second wheel head, which can be controlled independently of each other to be fed in the axial direction of a rotatably supported workpiece and in a direction perpendicular to the axial direction. The first wheel head includes a grinding wheel, and the second wheel head includes a superfinishing wheel. Axially separated object portions of the workpiece which have been ground by means of the grinding wheel are superfinished by means of the superfinishing wheel, while the workpiece is continuously supported on the grinding machine without the manner of support being changed. In this way, grinding and superfinishing are combined.
Grinding and superfinishing are performed on a single machine, while the workpiece is supported by means of a workpiece support mechanism without the manner of support being changed, so that transfer of the workpiece from a grinding station to a superfinishing station becomes unnecessary. The position of a contact point of the grinding wheel with respect to the workpiece is varied together with the second wheel head, which can be advanced and retreated in an accurately controlled manner. Thus, the surface roughness of the object portion which has been ground by means of the grinding wheel of the first wheel head can be improved while the geometrical profile of the object portion, such as roundness and cylindricity, is maintained as finished accurately by means of the grinding wheel. Thus, machining can be performed intensively, and superfinishing accuracy is improved. Preferably, each of the grinding wheel and the superfinishing grinding wheel comprises a layer of CBN grains or diamond grains, which features a long grinding life. Preferably, the grinding wheel and the superfinishing wheel are disposed on the first and second wheel heads, respectively, in such a manner so as to face each other.
Preferably, a numerical control unit for controlling the grinding machine is programmed in the following manner. The grinding wheel of the first wheel head, which has its home position at one end of a common path, initially grinds the object portion which is located far from the home position; i.e., the object portion which is located near the other end of the common path, at which the home position of the second wheel head is located. Subsequently, the grinding wheel sequentially grinds the remaining object portions while the first wheel head is returning to its home position. The superfinishing wheel of the second wheel head sequentially superfinishes the object portions which have been ground by means of the grinding wheel, while the second wheel head is moving toward the home position of the first wheel head, and in parallel with grinding which is performed by means of the grinding wheel. Grinding and superfinishing are performed in parallel with each other on at least a portion of object portions, thereby shortening machining time per workpiece. Preferably, the grinding wheel starts grinding from the object portion which is located nearest to the home position of the second wheel head, and then grinds the remaining object portions while the first wheel head is returning to its home position. Also, the superfinishing wheel superfinishes the object portions which have been ground by means of the grinding wheel, while moving after the first wheel head toward the home position of the first wheel head. Thus, total time required for grinding and superfinishing can be minimized.
Preferably, grinding of one object portion and superfinishing of one ground object portion are performed in parallel with each other. Superfinishing is performed in parallel with fine grinding which is performed upon completion of rough grinding of the object portion. Superfinishing is performed in parallel with fine grinding, which is performed under gentle grinding conditions, not in parallel with rough grinding, which is performed under severe grinding conditions Thus, the accuracy of a superfinished surface can be prevented from being affected by vibration and deflection of the workpiece which would arise if superfinishing were performed in parallel with rough grinding. In view of shortening of machining time, preferably, grinding and superfinishingxe2x80x94in which an object portion to be ground and a ground object portion adjacent to the object portion simultaneously undergo grinding and superfinishing, respectivelyxe2x80x94are performed while the first and second wheel heads are moving toward the home position of the first wheel head. Herein, xe2x80x9cfine grindingxe2x80x9d may be a final grinding step involving a final feed for cutting of the grinding wheel, or an intermediate grinding step to be performed between a rough grinding step and the final grinding step.
Preferably, upon completion of grinding of each of the object portions, the spatial relationship (i.e., relative position) between the object portion and the grinding wheel is stored. When superfinishing is to be started, the superfinishing wheel is accurately positioned at a superfinishing start position on the basis of stored position data. Since superfinishing start position is determined on the basis of data regarding a tool position as detected upon completion of grinding of each of the object portions, the superfinishing wheel can be finely fed for cutting against a ground surface of the object portion. In the case of a workpiece exhibiting low rigidity and marked anisotropy in a direction of rotation, such as a crankshaft, a grinding wheel encounters difficulty in effecting accurate, fine feed for superfinishing. The present invention solves this problem. Also, the above-described means for solution can shorten a so-called idle feed time which is required at the beginning of superfinishing in order to bring the superfinishing wheel into contact with a ground surface of each of the object portions at an appropriate feed speed for cutting. Thus, machining efficiency is improved.
The position of the grinding wheel upon completion of grinding can be detected by means of an output from an. absolute encoder, which constitutes the feed unit of the first wheel head and serves as position-detecting means. Alternatively, a contact point of the grinding wheel may be detected through use of optical or magnetic detection means. When superfinishing is to be performed, the superfinishing wheel is positioned on the basis of data regarding the position of the grinding wheel or the first wheel head as detected upon completion of grinding. Calculation of the superfinishing start position takes into account a diametral difference between the grinding wheel and the superfinishing wheel, and a positional difference between the first wheel head and the second wheel head with respect to the origin of the X-coordinate system; i.e., the machine origin, which is for example the axis of rotation of the workpiece. The numerical control unit may perform this calculation immediately after detection of the position of the grinding wheel or the first wheel head, or immediately before superfinishing is started.
Ablation of the grinding wheel derived from friction with the workpiece and that derived from truing which is performed for regeneration of the grinding surface of the grinding wheel can be detected through use of known detection means which involves direct or indirect contact with the grinding surface of the grinding wheel. However, in the case where high resolution of detection is required, the difference between the thus-detected apparent diameter and an actual diameter with respect to the grinding wheel becomes nonnegligible. In order for the numerical control unit to calculate an accurate superfinishing start position by rendering small an error derived from an apparent diameter of the grinding wheel, use of a grinding wheel of super abrasive is preferred, since a decrease in the diameter of such a grinding wheel is small over a long life.
Advantageously, a rest unit is provided in order to support the workpiece from opposite the grinding wheel at least when the grinding wheel performs rough grinding. However, the rest unit does not support the workpiece at least when the superfinishing wheel performs a final stage of superfinishing; specifically, final spark-out. At the final stage of superfinishing, elimination of mechanical friction between the workpiece and the rest shoe prevents chattering which would otherwise result from the mechanical friction, as well as an impairment in roughness of a superfinished surface of the workpiece.
Preferably, the present invention is applied to a crank-pin grinding machine of a C-X control system. The workpiece assumes the form of a crankshaft. The workpiece support mechanism supports the crankshaft such that the crankshaft rotates about the journal axis to thereby revolve crank pins around the journal axis. During grinding or subsequent superfinishing, the first or second wheel head advances or retreats synchronously with the phase angle of the revolving crank pin to be ground or superfinished.
Each of the crank pins is initially ground by means of the grinding wheel and is then superfinished by means of the superfinishing wheel. During grinding or superfinishing, rotation of the crankshaft and advancement/retreat of the first or second wheel head are synchronized. Thus, even though the rigidity of the crankshaft varies greatly with the angular position of rotation, the crank pins can be ground and superfinished highly efficiently and accurately. During grinding and subsequent superfinishing, support of the crankshaft effected by the workpiece support mechanism remains unchanged, whereby the superfinishing wheel can be finely fed for cutting against the ground surface of the crank pin. Preferably, immediately after each of the crank pins is ground, the crank pin is superfinished. Also, the position of the grinding wheel as detected upon completion of grinding is stored. On the basis of the position data, the superfinishing wheel is positioned before starting superfinishing. More preferably, grindingxe2x80x94which involves severe grinding conditions, such as a large grinding allowancexe2x80x94performed while the rest unit supports the crankshaft so as to prevent deflection of the crankshaft. Also, superfinishing; at least, the final stage of superfinishingxe2x80x94which involves a small grinding allowancexe2x80x94is performed while no mechanical contact is established between the crankshaft and the rest unit, thereby avoiding an adverse effect on final surface roughness which would otherwise result from the mechanical contact.
Advantageously, a large amount of coolant is supplied toward a region of grinding located between the grinding wheel and the object portion during grinding, which is performed under severe conditions. During superfinishing, which is performed so as to greatly improve profile irregularity, a small amount of coolant is supplied to the object portion, and oil mist serving as lubricant is sprayed over the superfinishing wheel. Decreasing of coolant supplied during superfinishing prevents chattering and generation of a dynamic pressure which would otherwise result from coolant caught between the object portion and the superfinishing wheel, thereby further improving a superfinished surface profile; i.e., geometrical accuracy and surface accuracy.
Preferably, a filter is employed for preventing entry of foreign matter into coolant supplied during superfinishing, and an amount of coolant supplied during superfinishing is not greater than {fraction (1/10)} that of coolant supplied to the region of grinding during grinding. This eliminates chattering and generation of a dynamic pressure and prevents formation of scratches on a superfinished surface.
Preferably, in the case where the workpiece assumes the form of a crankshaft, before or after all crank pins are ground and superfinished, all journals of the crankshaft are ground and superfinished. This enables grinding and superfinishing of journals and crank pins of the crankshaft on a single grinding machine through involvement of a single setup of the crankshaft on the machine, thereby further improving machining accuracy and efficiency. Preferably, grinding and superfinishing of journals are performed prior to grinding and superfinishing of crank pins, which involve complicated control, such as angular control of crankshaft rotation, thereby achieving high accuracy of finishing of crank pins.